As shown in FIG. 1, an existing 3T1C (3 transistors T1, T2, T3, and 1 capacitor Cst) pixel driving circuit for an Organic Light Emitting Diode (OLED) is illustrated, wherein D is a data driving signal; G is a charging scanning signal; DG is a discharging scanning signal; ODdd is a constant voltage signal; Ovss is an active OLED output voltage; and Vref is a reference voltage. When the circuit operates for digital driving, only two Gamma voltage levels, i.e., both GM1 (brightest) and GM9 (darkest) voltage levels, are output at VA. A transistor current-voltage (I-V) equation is expressed by:Ids,sat=k·(VGS−Vth,T2)2=k·(VA−VS−Vth,T2)2,
wherein Ids,sat is a transistor activation current; k is an intrinsic conduction factor; VGS is a transistor gate-source voltage; Vth,T2 is a threshold voltage of transistor T2; VA represents a voltage of point VA; and VS represents a voltage of point VS. Compared with an analog driving mode, the digital driving mode may restrain the problem of non-uniform luminance of the OLED, because the change ΔVth of a transistor threshold voltage Vth is small relative to (VA-VS) due to device degradation or inconsistency.
When the pixel driving circuit as shown in FIG. 1 works, transistor T1 charges for the voltage at point VA, while transistor T3 discharges for the voltage at point VA; eventually, only two Gamma voltage levels are controlled to be output at VA, and gray levels are sliced out in a Pulse-Width Modulation (PWM) way.
FIG. 2 shows a diagram of using PWM driving for subfields of 6 bits, and 1280 scanning lines. Images different in gray level luminance may be displayed using digital voltages (i.e., two Gamma voltages) by controlling the length of subfield (SF) charging time in combination with the principle that human eye perception of luminance is integral of time domain. The subfields are displayed by time in an order of bit 0, bit 1, bit 2, bit 3, bit 4, and bit 5; the weights of the subfields are 1:2:4:8:16:32; oblique line 1 is the course of enabling a pixel charging scanning line; Tch is the time of charging a complete pixel within a subfield; oblique line 2 is the course of enabling a pixel discharging scanning line; and Tdch is the time of discharging a complete pixel within a subfield.
With the 6 subfields different in weight as shown in FIG. 2 for example, if subfields of one frame of image are driven according to weight ratio of 1:2:4:8:16:32, FIG. 3 shows relations between various gray levels and the subfields, wherein circles represent opening of corresponding subfields. When one frame of image is switched to another frame of image and gray levels of pixels are switched from 3 to 4, 7 to 8, 15 to 16, 31 to 32, 32 to 31, etc., wrong dark lines or bright lines will appear in positions where the gray levels of pixels are switched, as shown in FIG. 4, thus leading to an image display error.